The present invention relates to an outer-rotor type brushless motor provided with an index detection mechanism.
Outer-rotor type brushless motors are used as spindle motors for rotationally driving color wheels in projectors, polygon mirrors in printers and the like. Brushless motors of this sort are known in which an index detection mechanism is provided, which outputs a position signal of one pulse for each rotation (index signal) so as to detect the rotational position of the rotor.
For example, in the brushless motor described in Japanese Patent Laid-Open Publication No. 2001-339889, windows are provided at multiple locations on a rotor yoke, and protrusions are provided on a portion of a ring-shaped drive magnet that fits in the inner periphery of the rotor yoke, which protrude radially outward through the windows. Here, one of these protrusions is provided at a position that straddles a north and south pole of the drive magnet, and a magnetic flux detection element is arranged at a position facing the protrusion.
With the brushless motor described in Japanese Patent Laid-Open Publication No. 2001-339889, when the rotor yoke rotates, the magnetic flux from the protrusion is detected by the magnetic flux detection element, and an index sign& is output consisting of one pulse for each rotation of the rotor yoke.
Furthermore, in the brushless motor described in Japanese Patent Laid-Open Publication No. 2001-339889, because a window is provided in the rotor yoke and a portion of the drive magnet protrudes radially outward from this window, the motor efficiency tends to decrease if this window is overly large. Consequently, the one protrusion mentioned above is provided at a position straddling the north and south poles of the drive magnet, formed within a narrowly limited range from the border between the north and south poles (a range of approximately ¼ of each of the magnetic poles).
However, because the magnetic flux density is lower in the region of the border between the north and south poles than in the circumferentially central regions of the poles, the magnetic flux that is detected by the magnetic flux detection element tends to be influenced not only by changes in the magnetic field of the drive magnet, which depends on the ambient temperature, but also by variations in the relative positions of the protrusion and the magnetic flux detection element, and by magnetic disturbances, Consequently, in order to obtain a highly reliable index signal there is a need for high dimensional accuracy in the multiple components that make up the motor, and a need for highly accurate positioning of the protrusion and the magnetic flux detection element, which is a problem in so much as this increases component costs and lowers assembly work efficiency.